1. Field of the Invention
This invention relates to a high-frequency attenuator using a PIN diode.
2. Background
In a high-frequency attenuator using a PIN diode, the PIN diode's attenuation level is adjusted by changing the bias current supplied to the PIN diode. This in turn changes the high- frequency resistance of the PIN diode.
FIG. 1 shows an example of the high-frequency attenuator using a PIN diode. In FIG. 1, a first capacitor Cp, PIN diode D and second capacitor Cp are serially connected in this order between input and output terminals. One end of a choke coil Lc is connected to the anode of the PIN diode and current I is supplied through the choke coil Lc to the PIN diode. The high-frequency impedance of the attenuator changes according to the current I. The other end of the choke coil is grounded through a third capacitor Cp. The cathode of the PIN diode D is grounded through a second choke coil Lc. The three capacitors are used to block direct current.
Curves A and B in FIG. 2 show the frequency characteristics of the attenuator shown in FIG. 1 when the current I is equal to 10 mA and 0 mA respectively. As shown by curve A, when the current I supplied to the PIN diode is 10 mA, the attenuation-frequency characteristics over the frequencies of interest are nearly flat. When the current I is not supplied to the PIN diode D as shown by curve B, the PIN diode D is cut off. The attenuation level for higher frequency signals is smaller than that for lower frequency signals in curve B.
A high frequency attenuator as explained above may be used in a transmitter/receiver system of a cellular phone, for example. FIG. 3 shows one example of such a cellular phone system using a high frequency attenuator. In FIG. 3, a baseband signal to be transmitted is supplied to a modulator 1 where the baseband signal is converted to an intermediate frequency band signal. The intermediate frequency band signal is, then, further converted to a high frequency signal in a radio frequency band by a local oscillator 2 and a mixer 3. The high frequency signal is amplified by an amplifier 4 and supplied to a PIN diode high frequency attenuator 5. The high frequency attenuator 5 attenuates the high frequency signal to a desired extent. The output of the high frequency attenuator 5 is then power amplified by a power amplifier 6. The output of the power amplifier 6 is supplied to a power detector 7 which detects the power at the output of the power amplifier 6.
The output of the power detector 7 is supplied to a power controller 8. Power controller 8 controls the attenuation level of the high frequency attenuator 5 according to the power detected by the power detector 7. This forms a feedback circuit comprising the power detector 7 and the power controller 8. This feedback system stabilizes the output power of the power amplifier to compensate for fluctuations in power supply voltage and operating temperature. The output of the power amplifier 6 is also supplied to a duplexer 9 and finally transmitted from an antenna 10.
In the receiver system of FIG. 3, a high frequency signal is received by the antenna 10 and supplied to a high frequency amplifier 11 through the duplexer 9. After being amplified by the high frequency amplifier 11, the high frequency signal is frequency converted to an intermediate frequency band signal by first and second stages of frequency conversion. The first stage includes a local oscillator 12, a mixer 13 and an amplifier 14. The second stage includes a local oscillator 15, a mixer 16 and an amplifier 17. The intermediate frequency band signal from the amplifier 17 is then demodulated to a baseband signal by a demodulator 18.
In the above described cellular phone system, the high frequency attenuator 5 using PIN diode is used to control the power of the signal transmitted from the antenna 10 so that the power of the transmitted signal is kept within a range required by standards or regulations.
For example, under North American cellular phone system standards, the power of the high frequency signal transmitted from an antenna has to be variable from +36 dBm to +8 dBm. Since power amplifier 6 has a constant gain, the high frequency attenuator needs a variable attenuation range which is more than 28 dB over the operating frequency band (for example, the 800 MHz band).